In many fields, it is indispensable to detect each load (its force and moment) applied to a body or a specific portion of the body. When effecting for example assembly work or grinding/deburring work by means of a high-performance robot, it is necessary to detect precisely loads applied to the hand of the robot. When conducting a model test for aircraft, shipping, vehicle or the like, the detection of loads exerted to various parts also becomes principal test items.
As sensors excellent for use in the detection of such loads, there have been proposed sensors each making use of a parallel plate structure for detecting force components only in the direction of a standard axis and sensors each of which employs a radial plate structure capable of detecting moment components about a standard axis. Such parallel and radial plate structures will hereinafter be described with reference to certain drawings.
FIG. 21 is a perspective view of a planar flexible beam which is a component of a parallel or radial plate structure. In the drawing, numeral 39 indicates a support portion and numeral 40 designates a planar flexible beam supported in a cantilevered fashion on the support portion 39. Let's now set up axes X, Y and Z, which are perpendicular to each other, as shown in the drawing. Let's thus represent the force components along the axes X, Y and Z as well as the moment components about the same axes at the tip portion of the planar flexible beam 40 as F.sub.X, F.sub.Y and F.sub.Z as well as M.sub.X, M.sub.Y and M.sub.Z respectively. The planar flexible beam 40 is thinner in the direction of the Z-axis. Its dimensions in the directions of the X-axis and Y-axis are significantly greater than its thicknesswise dimension. Accordingly, it is susceptible of developing bending deformation by the force F.sub.Z. It is also susceptible of developing deformation by the moment M.sub.Y which causes the tip portion of the planar flexible beam 40 to undergo movement in the direction of the Z-axis. On the other hand, the planar flexible beam 40 is extremely resistant to deformations by the forces F.sub.X,F.sub.Y and moment M.sub.Z. The susceptibility of deformation by the moment M.sub.X lies between those by the force F.sub.Z and moment M.sub.Y and those by the forces F.sub.X,F.sub.Y and moment M.sub.Z, and may be ignored or may not be ignored depending on the dimensions of the planar flexible beam 40 or the extent of deformation under consideration. The parallel plate structures and radial plate structures have been constructed on the basis of these characteristics of the planar flexible beam 40.
FIG. 22(a) through FIG. 22(c) are side views of a parallel plate structure. In each of these figures, there are illustrated a fixed portion 41 supported on the support portion 39 and made of a rigid material and a movable portion 42 located opposite to the support portion 39 and made of a rigid material. Numerals 43,43' indicate thin-walled portions which connect the fixed portion 41 and the movable portion 42 to each other. These thin-walled portions 43,43' are arranged parallel to each other and have deformation characteristics similar to the planar flexible beam 40 shown in FIG. 21. Designated at numeral 44 is a parallel plate structure, which has a shape obtained by boring a square hole through a rigid body because thin-walled portions 43,43' are arranged in parallel to each other therein. It should, however, be borne in mind that the shape of the hole, which defines the thin-walled portions 43,43', is not necessarily limited to such a square shape, as will be described hereinafter. Letter K indicates the standard axis of the parallel plate structure 44, which standard axis extends through the movable portion 42 of the parallel plate structure 44. The standard axis K is located at a suitable distance from the thin-walled portions 43,43' and is near a point at which the below-described force F.sub.Z is principally applied. Designated at numerals 45,46,47, 48 are strain gauges provided respectively at end parts of the thin-walled portions 43,43'.
When the force F.sub.Z is applied in the direction of the Z-axis to the movable portion 42 of the parallel plate structure 44, the thin-walled portions 43,43' of the parallel plate structure 44 undergo bending deformations of the same pattern as depicted in FIG. 22(b). These deformations occur readily because, as mentioned above, the planar flexible beam 40 of FIG. 21 is susceptible to bending deformation to the force F.sub.Z applied in the direction of the Z-axis and moreover, the deformations of the thin-walled portions 43,43' are of the same pattern and their mutual interference is limited to a small extent.
Let's discuss the deformation of the parallel plate structure when the moment M.sub.Y is applied to the movable portion 42. The planar flexible beam 40 of FIG. 21 is by itself susceptible to bending deformation by the moment M.sub.Y. But, as for the parallel plate structure 44 composed of two planar flexible beams only, the deformation mode caused by the moment M.sub.Y is such that the thin-walled portion 43 is elongated while the thin-walled portion 43' is compressed as shown in FIG. 22(c). This deformation makes the length of the thin-walled portions 43,43' different from each other and, correspondingly, produces a pair of large internal stresses of different directions along the direction of the X-axis within the thin-walled portions 43,43' respectively. Therefore, the moment M.sub.Y has to be of a very large magnitude in order to produce such deformation. In other words, the parallel plate structure 44 has a very high rigidity against the moment M.sub.Y.
Although the twisting deformations of the thin-walled portions 43,43' by the moment M.sub.X are smaller compared with their bending deformations by the force F.sub.Z and moment M.sub.Y as aforementioned on the planar flexible beam 40 of FIG. 21, their extents are not small enough to permit them to be ignored compared with the bending deformations of the thin-walled portions 43,43' as mentioned above. The thin-walled portions 43,43' have been rendered sufficiently rigid against the moment M.sub.X by arranging them into the parallel plate structure. However, the influence of twisting deformations cannot be ignored when a still greater moment, M.sub.X, is applied. Even in this case, it is still possible to get rid of the influence of twisting deformations if the strain gauges 45,46,47,48 of FIG. 22 are provided at the thin-walled portions 43,43' in the middle of dimension perpendicular to the drawing sheet, because no strains are produced there by such twisting deformations. By the way, it is apparent that the rigidity against the forces F.sub.X,F.sub.Y and moment M.sub.Z are sufficiently high as the thin-walled portions 43,43' have by themselves high rigidity against such forces and moment.
From the above, it has been understood that the parallel plate structure 44 of FIG. 22 undergoes a significant deformation only by the force F.sub.Z and is very rigid against the other forces and moments. Namely, it is appreciated that this parallel plate structure 44 is suitable for use as a force detection element adapted to detect the force component F.sub.Z only out of a given load.
Here, description is made on the detection of strains of the thin-walled portions 43,43' by the strain gauges 45,46,47,48. When the force F.sub.Z is applied as shown in FIG. 22(b), tensile strains are produced in the strain gauges 45,48 while compression strains are developed in the strain gauges 46,47. When the forces F.sub.X,F.sub.Y and moments M.sub.X,M.sub.Y,M.sub.Z are exerted concurrently with the force F.sub.Z, the strain gauges 45, 46,47,48 are not affected by F.sub.X,F.sub.Y,M.sub.Z as the thin-walled portions 43,43' are substantially rigid against them. However, the strain gauges 45,46,47,48 are deformed by M.sub.X,M.sub.Y in modes corresponding thereto, although their deformations are slight. It has already been mentioned that the influence of M.sub.X can be overcome by making suitable selection as to the positions where the strain gauges are arranged. However, a deformation mode such as that depicted in FIG. 22(c) is developed by M.sub.Y to a certain extent. The strain due to deformation by the force F.sub.Z can be enlarged and the small output component corresponding to the deformation by the moment M.sub.Y can be cancelled out, provided that a bridge circuit is constructed in such a way that the outputs of the strain gauges 45,48 are added together, the outputs of the strain gauges 46,47 are also added together, and that the subtracted signal of the thus-added values is put out. In this manner, a correct signal proportional to the force F.sub.Z can be obtained.
As these detection means, there are other strain detection means such as magnetic strain elements besides strain gauges. Similar detection is feasible with such other magnetic strain elements. It is also possible to make up detection elements for the force F.sub.Z by using differential transformers and electrocapacitive or eddy-current displacement detection elements for the detection of displacement of the movable portion 42 in the Z-direction while making use of the characteristic feature that the parallel plate structure 44 undergoes a significant deformation only by the force F.sub.Z.
FIG. 23(a) through FIG. 23(c) are side views of a radial plate structure. In each of the figures, numeral 51 indicates a fixed portion supported on the support portion 39 and made of a rigid material, while numeral 52 designates a movable portion located opposite to the support portion 39 and made of a rigid material. Designated at numerals 53,53' are thin-walled portions which connect the fixed portion 51 and the movable portion 52 to each other. These thin-walled portions 53,53' extend radially at a crossing angle .theta., starting at a point O from the movable portion 52 toward the fixed portion 51. Each of the thin-walled portions 53,53' has deformation characteristics equivalent to that of the planar flexible beam 40 illustrated in FIG. 21. By the way, this crossing angle .theta. is set at an acute angle for the convenience of explanation. Designated at numeral 54 is a radial plate structure, which has a shape formed by boring a trapezoidal hole through a rigid body because the thin-walled portions 53,53' are arranged radially. It should however be borne in mind that the shape of the hole defining the thin-walled portions 53,53' is not necessarily limited to such a trapezoidal shape, as will be described hereinafter. Letter K indicates an axis which extends through the point O on the movable portion 52 of the radial plate structure 54 and perpendicularly to the drawing sheet. The axis K serves as the standard axis of the radial plate structure 54. Numerals 55,56,57,58 are strain gauges provided respectively at end parts of the thin-walled portions 53,53'.
When a moment M.sub.Y is applied about the Y-axis to the movable portion 52 of the radial plate structure 54, the thin-walled portions 53,53' of the radial plate structure 54 undergo bending deformations of substantially the same pattern as illustrated in FIG. 23(b). These deformations occur readily, since as mentioned above, the planar flexible beam 40 of FIG. 21 is susceptible of undergoing a bending deformation by a force applied at a right angle thereto and the deformations of the thin-walled portions 53,53' are of substantially the same pattern and the degree of their mutual interference is small.
A further discussion will be made on the deformation of the radial plate structure 54 when a force F.sub.Z is exerted to the movable portion 52. Although the planar flexible beam 40 of FIG. 21 is by itself susceptible to a bending deformation by the force F.sub.Z, the radial plate structure 54 composed by combining two parallel beams, each being of the same type as the planar flexible beam 40, has such a deformation mode for the force F.sub.Z that the thin-walled portion 53 is elongated and the thin-walled portion 53' is compressed as shown in FIG. 23(c). This deformation renders the lengths of the thin-walled portions 53,53' different from each other and correspondingly, produces a pair of large internal axial stresses of opposite directions within the thin-walled portions 53,53' respectively. Therefore, the force F.sub.Z has to be very large in order to develop such a deformation. In other words, the radial plate structure 54 has a very high rigidity against the force F.sub.Z.
Although the twisting deformations of the thin-walled portions 53,53' by the moment M.sub.X are smaller than their bending deformations as aforementioned on the planar flexible beam 40 of FIG. 21, their twisting deformations are not of such extents as being successfully ignorable compared with their bending deformations as mentioned above. However, the thin-walled portions 53,53' can be rendered sufficiently rigid against the moment M.sub.X by forming them into a radial plate structure. When a still greater moment M.sub.X is applied, the influence of the twisting deformations becomes no longer ignorable. In this case, it is possible, as mentioned above, to remove the influence of the twisting deformations by providing the strain gauges 55,56,57,58 on the thin-walled portions 53,53' in the middle of dimension perpendicular to the drawing sheet because no strain would be produced there by twisting deformations. By the way, it is apparent that the radial plate structure 54 has sufficiently high rigidity against the forces F.sub.X,F.sub.Y and moment M.sub.Z, since the thin-walled portions 53,53' have by themselves high rigidity against such forces and moment.
For the reasons mentioned above, it has been found that the radial plate structure 54 of FIG. 23 undergoes a significant deformation only by the moment M.sub.Y but is very rigid against the other moments and forces. Namely, it is appreciated that this radial plate structure 54 is most suitable as a moment detection element capable of detecting only the moment component M.sub.Y out of a given load.
Here, description is made on the detection of strains of the thin-walled portions 53,53' by the strain gauges 55,56,57,58. When the moment M.sub.Y is applied as shown in FIG. 23(b), tensile strains are produced in the strain gauges 55,58 while compression strains are developed in the strain gauges 56,57. When the forces F.sub.X,F.sub.Y,F.sub.Z and the moments M.sub.X,M.sub.Z are exerted concurrently with the moment M.sub.Y, the strain gauges 55, 56,57,58 are not affected by the forces F.sub.X,F.sub.Y and the moment M.sub.Z. The radial plate structure 54 is substantially rigid against the forces F.sub.X,F.sub.Y and the moment M.sub.Z, but it undergoes slight deformations by the force F.sub.Z and the moment M.sub.X as mentioned above. It has already been mentioned that the influence of M.sub.X can be overcome by making suitable selection as to the positions where the strain gauges are arranged. However, a deformation mode such as that depicted in FIG. 23(c) is slightly developed by the force F.sub.Z. The strain due to deformation by the moment M.sub.Y can be enlarged and the small output component corresponding to the deformation by the force F.sub.Z can be cancelled out, provided that a bridge circuit is constructed in such a way that the outputs of the strain gauges 55,58 are added together, the outputs of the strain gauges 56,57 are also added together, and that the subtracted signal of the thus-added values is put out. In this manner, a correct signal proportional to the moment M.sub.Y can be obtained.
In the above description, the crossing angle .theta. between the thin-walled portions 53,53' is set at an acute angle for the sake of convenience. If it is an obtuse angle, the rigidity of the radial plate structure 54 against the respective moments M.sub.X,M.sub.Z mentioned above, will be reversed. If the crossing angle .theta. is 90 degree, its rigidity against the moment M.sub.X and that against the moment M.sub.Z will be equal to each other. Whether those rigidities against the moments M.sub.X,M.sub.Z are different or the same, the radial plate structure 54 will undergo slight twist deformation by one or both of those moments, when they are large. For the elimination of such influence, the strain gauges 55,56,57,58 may be arranged on the thin-walled portions 53,53' in the middle of the dimension perpendicular to the drawing sheet as mentioned above.
FIG. 24(a) through FIG. 24(d) are side views of another parallel plate structure and radial plate structure. In these figures, like reference numerals and letters identify like elements of the parallel plate structure shown in FIG. 22(a) through FIG. 22(c) and those of the radial plate structure depicted in FIG. 23(a) through FIG. 23(c). In FIG. 24(a), numeral 49 indicates a parallel plate structure. FIG. 24(b) illustrates a deformation which takes place when the F.sub.Z force is applied in the direction of the Z-axis to the standard axis K of the parallel plate structure 49. In FIG. 24(c), numeral 59 indicates a radial plate structure. FIG. 24(d) illustrates a deformation which takes place when the moment M.sub.Y is applied about the standard axis K of the radial plate structure 59. The parallel plate structure 44 and the radial plate structure 54 shown respectively in FIG. 22(a) and FIG. 23(a) exhibit better characteristics when they are formed into symmetrical structures relative to their vertical central axes as depicted in FIGS. 24(a) and 24(c). Although all the characteristic features which have been described above are contained in the parallel plate structure 49 and the radial plate structure 59, such symmetrical structures stabilize the deformation modes typical to the respective structures and permit better performance. In FIG. 22(a), the definition for the standard axis K of the parallel plate structure 44 is not clear. However, it is clear in FIG. 24(a). Namely, the standard axis K is an axis which extends through the center of the movable portion 42 and also at equal distances from the centers of the thin-walled portions 43,43' in a direction perpendicular to the thin-walled portions 43,43'. The standard axis K of the radial plate structure 59 is exactly the same as that illustrated in FIG. 23(a).
FIG. 25(a) through FIG. 25(d) are side views of further parallel plate structures and radial plate structures. In these figures, like reference numerals and letters identify like elements of structures illustrated in FIG. 22(a) and FIG. 23(a). In FIG. 25(a), numeral 64 indicates a circular hole bored through a rigid body. By this circular hole 64, the thin-walled portions 43,43' of the parallel plate structure are defined. The right half part of the parallel plate structure is omitted in the figure [this also applies to FIGS. 25(b) through 25(c)]. In FIG. 25(b), numeral 65 indicates small circular holes bored respectively through upper and lower edge portions of a rigid body in a precisely opposed relation, and numeral 66 designates a straight slot extending between these two circular holes 65,65. By these holes 65 and the slot 66, the thin-walled portions 43,43' of the parallel plate structure are defined. In FIG. 25(c), designated at numeral 67 is a substantially elliptical hole bored through a rigid body and extending close to the upper and lower edge portions of the rigid body. By this hole 67, the thin-walled portions 53,53' of the radial plate structure are defined. Turning next to FIG. 25(d), numeral 68 indicates small circular holes bored in upper and lower edge portions of a rigid body in a precisely-opposed relation, and numeral 69 designates a straight slot extending between these two circular holes 68,68. The thin-walled portions 53,53' of the radial plate structure are defined by these holes 68,68 and the slot 69.
As described above, the thin-walled portions 43,43',53,53' shown respectively in FIGS. 25(a) through 25(d) are not uniform in longitudinal thickness, whereby it is of course understood that the forms of the portions 43,43';53,53' are not symmetrical. However, they still exhibit the same effects as the parallel plate structures shown respectively in FIG. 22(a) and FIG. 24(a) and the radial plate structures depicted in FIGS. 23(a) and 24(c). By the way, the thin-walled portions 43,43',53,53' are not uniform in thickness as described above. It is therefore not fully appropriate to call them plates. However, these thin-walled portions 43,43',53,53' have the same flexible function as the planar thin-walled portions depicted in FIGS. 22(a), 23(a), 24(a) and 24(c). No problems will thus arise when the thin-walled portions 3,43',53,53' are considered to be equivalent to the planar thin-walled portions shown in these figures. Accordingly, thin-walled portions having non-uniform wall thicknesses such as those depicted in the above-mentioned figures will hereinafter be deemed as planer thin-walled portions. Hence, the terms "parallel plate structure" and "radial plate structure" should be interpreted to embrace therein parallel plate structures and radial plate structures having such non-uniform wall thicknesses, besides those having uniform wall thicknesses.
Certain basic structures and characteristic properties of a parallel plate structure as well as those of a radial plate structure have been described above. Inventions making use of such plate structures are disclosed in Japanese Patent Publication No. 7657/1982 and Japanese Patent Laid-open No. 88631/1983. Namely, Japanese Patent Publication No. 7657/1982 discloses a 3-axis load meter equipped with load detection elements, each of which makes use of a parallel plate structure, arranged respectively along the X-axis, Y-axis and Z-axis. On the other hand, Japanese Patent Laid-open No. 88631/1983 discloses a thrust/torque meter constructed by arranging a parallel plate structure and a radial plate structure of a special shape with their standard axes coincided. They may be considered as sorts of multi-axis load sensors. However, they all served to detect only the direction and magnitude of each force or to detect force and moment separately. They were unable to detect the point of action of an applied force. With a view toward additionally imparting this function, it has been proposed in the specification and drawings of Japanese Patent Laid-open No. 62497/1985 (corresponding to U.S. patent application Ser. No. 605,212 of Apr. 30, 1984 and European Patent Application No. 84200591.0) to provide a multi-axis load sensor which makes use of one or more parallel plate structures and at least one radial plate structure. Such a sensor can detect not only the direction and magnitude of each load, but also its point of action. Compared with the special radial plate structure disclosed in Japanese Patent Laid-open No. 88631/1983 and composed of planar plates arranged with an equal angular interval, the moment detection element depicted in FIG. 24(a) has such a new characteristic feature as it is easy to form a laminated structure, although the special radial plate structure and the moment detection element are coincided in their principal functions that they detect moments. Such a moment detection element has also been proposed in the specification and drawings of Japanese Patent application No. 162527/1983, which was laid open under No. 55239/1985.
The above-described various multi-axis load sensors, each of which employs one or more parallel plate structures and one or more radial plate structures, are each satisfactorily useful for actual applications and are thus actually used in apparatus. They are, however, accompanied by the following shortcomings.
(1) It is necessary to provide parallel or radial plate structures as many as the number of force and/or moment components to be detected. If there are as many as 5 or 6 axes, it becomes usually difficult to combine such many plate structures into a unit.
(2) The multi-axis load sensor shown in the above referred-to Japanese Patent Laid-open No. 62497/1985 (corresponding to U.S. patent application Ser. No. 605,212 of Apr. 30, 1984 and European Patent Application No. 84200591.0) has ingeniously materialized the above combination. However, it requires a high machining cost.
(3) When many load components are taken into consideration, deformations at the detecting parts of the respective components may be summed up to such an extent that the resulting overall deformation cannot be ignored, although such deformations may be small when taken individually.